This paper presents an efficient algorithm for finding the dominant trapping sets of irregular low-density parity-check (LDPC) codes. The algorithm can be used to estimate the error floor of irregular LDPC codes or to be part of the apparatus to design irregular LDPC codes with low error floors. The algorithm is initiated with a set of short cycles, variable nodes with low degree, and cycles with low approximate cycle extrinsic message degree (ACE), as the input. The input structures are then expanded recursively to dominant trapping sets of increasing size. The algorithm is devised based on the careful inspection of the graphical structure of dominant trapping sets and the relationship of such structures to short cycles, low-degree variable nodes and cycles with low ACE. In particular, the important role of degree-2 variable nodes in the structure of dominant trapping sets is discussed. Simulation results on several LDPC codes demonstrate the accuracy and efficiency of the proposed algorithm. In particular, the algorithm is significantly faster than the existing search algorithms for dominant trapping sets.
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